Houston-based Enbridge Energy Inc.'s pipeline carries crude oil from Saskatchewan, Canada, to the Chicago area. It carries roughly 16 percent of U.S. crude imports, and oil prices shot up more than $4 early Thursday when word of the fire first reached oil markets.
The pinhole leak was fixed with a repair sleeve earlier this month.
On Wednesday, workers shut down the pipeline to remove the 11-foot (3.4-meter) section that included the pinhole and sleeve. They replaced it with a new section of pipeline, but oil apparently leaked at a section where that joined the old line, said Leon Zupan, Enbridge's vice president of operations.
Three of the four lines were operating by Thursday afternoon. The company said the fourth line, where the fire occurred, would probably be usable again in two or three days.
The company's metallurgists want to analyze the section to better understand why it leaked, said Enbridge spokesman Larry Springer. Electronic tools were put inside the pipeline in 2006 looking for dents and metal loss.
"There were no problems found in that area where the leak occurred," he said.
The killed workers were identified by Enbridge on Thursday as Dave Mussati Jr. and Steve Arnovich, both contract workers from its Superior, Wisconsin, office.
The area where the fire occurred, Clearbrook, is about 215 miles (345 kilometers) northwest of Minneapolis.
The crude is used to make several kinds of fuel, such as gasoline and home heating oil.